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Marine Predator Diets and Food Web Indicators

Variable ocean conditions can greatly impact lower trophic level prey assemblages in marine ecosystems, with effects propagating up the food chain. We are studying how varying ocean conditions affect long term changes in seabird diets and dietary niche overlap among a suite of low- to mid trophic predators. One central question is whether we can use predator diets as a method to track prey communities. We studied the diets of common murres (Uria aalge) over 10 contrasting years between 1998 and 2011, a period in which the Northern California Current experienced dramatic interannual variability in ocean conditions. We also collected diet samples from three predatory fish species on the central Oregon coast in collaboration with commercial and recreational fishermen during two summers of contrasting El Niño (2010) vs. La Niña (2011) conditions. We found greater niche specialization during El Niño and increased niche overlap during La Niña in both the nekton and micronekton diet components, especially in the larger, more offshore predators. However, only the two smaller, more nearshore predators exhibited interannual variation in diet composition. Concurrent trawl surveys confirmed that changes in components of predator diets reflected changes in the prey community. Analysis of murre diets for the 10-yr time series showed consistent dietary responses during warm and cold water periods. Using multiple predators across diverse taxa and life histories provided a comprehensive understanding of food-web dynamics during changing ocean conditions.